A continuously variable transmission (CVT) which can continuously change a gear ratio is known. In the continuously variable transmission, for example, a metal belt or a chain is held by each of a primary pulley and a secondary pulley. Also in the continuously variable transmission, the gear ratio is changed generally by changing the hydraulic pressure supplied to the primary pulley, and more specifically, the hydraulic pressure supplied to a primary sheave of the primary pulley.
For example, as the hydraulic pressure supplied to the primary sheave of the primary pulley is increased, the groove width of the primary pulley is narrowed. Consequently, the effective diameter of the primary pulley is increased. In accordance with an increase in the effective diameter of the primary pulley, the groove width of the secondary pulley is increased and the effective diameter thereof is decreased, with the result that the continuously variable transmission upshifts.
In contrast, as the hydraulic pressure supplied to the primary sheave of the primary pulley is decreased, the groove width of the primary pulley is increased. Consequently, the effective diameter of the primary pulley is decreased. In accordance with a decrease in the effective diameter of the primary pulley, the groove width of the secondary pulley is narrowed and the effective diameter thereof is increased, with the result that the continuously variable transmission downshifts.
The gear ratio of the continuously variable transmission is controlled based on the accelerator pedal position and the like. Japanese Patent Laying-Open No. 5-85228 (Patent Document 1) discloses a hierarchically configured electronic system in which a target output torque is determined based on the accelerator pedal position and the brake signal, and the target gear ratio is determined based on the determined target output torque and the vehicle speed.
Furthermore, the gear ratio of the continuously variable transmission is generally controlled such that the input shaft revolution speed reaches the set target value. The target value of the input shaft revolution speed is set in accordance with various control modes. For example, in the normal control mode, the target value of the input shaft revolution speed is set based on the information such as an accelerator pedal position, a vehicle speed, a brake signal, and the like. In the control mode in which a driver operates a shift lever or a paddle switch to request an upshift or a downshift, the target value of the input shaft revolution speed is set to be increased in proportion to the vehicle speed. In the control mode in which the target value of the input shaft revolution speed is set in accordance with the road on which the vehicle travels, the target value of the input shaft revolution speed is appropriately set based on the gradient of the road and the information of the corner (shape and the like of the curve) from the navigation system mounted in the vehicle. In addition, various control modes for setting the target value of the input shaft revolution speed are being put into practical use. The target value that is to be finally used is selected from the target values that are set in these various control modes. In other words, the target value that is to be finally used is selected by arbitrating the target value that is set in each control mode.